Recently, organic optical devices capable of being utilized in solar battery, electronic paper, electrophotography, etc have been intensively investigated. Of these, especially, electrophotographic technology has been widely used and applied not only in the field of copying machines but also in the field of various printers and printing machines in recent years because the technology is excellent in immediacy and can produce high-quality images.
As an electrophotographic photoreceptor (hereinafter, optionally abbreviated as “photoreceptor”) that is the core of the electrophotography technology, photoreceptors using inorganic photoconductors such as selenium, arsenic-selenium alloy, and zinc oxide have been conventionally employed but recently, the mainstream becomes the photoreceptors using organic photoconductive materials, which have the advantages of entailing no pollution, ensuring easy film formation and manufacture, having high freedom in material selection and combination, etc.
The sensitivity of the electrophotographic photoreceptors using organic photoconductive materials varies depending on the wavelength of exposure light and the kind of the charge generation substance.
As the charge generation substance having sensitivity to long-wavelength light of 600 to 800 nm, phthalocyanine compounds have been attracting attention. Especially, intensive studies have been conducted on metal-containing phthalocyanines such as chloroaluminum phthalocyanine, chloroindium phthalocyanine, oxyvanadium phthalocyanine, hydroxygallium phthalocyanine, chlorogallium phthalocyanine, magnesium phthalocyanine, and oxytitanium phthalocyanine, as well as metal-free phthalocyanines and the likes.
With regard to the phthalocyanine compounds, it is reported that even if the structure of the individual molecule is identical, a phthalocyanine compound may different in charge generation efficiency according to the regularity (crystal form) in arrangement of crystal, which is an agglomerate of molecules (see Non-Patent Documents 1 and 2).
In recent years, as the electrophotographic process in copying machines, laser printers, plain paper faxes, and the like has been advanced to be high-speed and full-colored one, it is essential to have high sensitivity and high-speed responding ability, so that it is inevitable to develop a more highly sensitive charge generation substance.
For high sensitivity, it is essential to have charge generation substances having a high charge generating ability. Of these, eager studies have been conducted on oxytitanium phthalocyanine exhibiting high sensitivity to LD exposure that is the current mainstream. The above oxytitanium phthalocyanine is known to show crystal polymorphism. As known crystal forms, there have been reported a large number of crystal forms such as α-form (see Patent Document 1), β-form (see Patent Document 2), C-form (see Patent Document 3), D-form (see Patent Document 4), Y-form (see Patent Document 5), M-form (see Patent Document 6), M-α-form (see Patent Document 7), and I-form (see Patent Document 8).
Among these crystal forms, a crystal form having a main peak at Bragg angle (2θ±0.2°) of 27.2° toward CuKα characteristic X-ray (wavelength 1.541 angstrom) (hereinafter, optionally referred to as “particular crystal form” in some cases) is known to exhibit high quantum efficiency and high sensitivity.
Moreover, other than the crystal composed of oxytitanium phthalocyanine molecule alone, mixed crystals composed of oxytitanium phthalocyanine and the other phthalocyanine or other pigment or the like are also widely known to form the above particular crystal form and exhibit high sensitivity (see Patent Document 9).
The above phthalocyanine crystals containing oxytitanium phthalocyanines having the particular crystal form are known to have very high sensitivity. It is considered that the high sensitivity is exhibited because water molecules are present in the crystals and function as sensitizers. However, the water molecules acting as sensitizers freely come in and out the crystals depending on humidity change in the environment of the crystals and hence the phthalocyanine crystals have a disadvantage that the water molecules may be eliminated from the crystals and bring about decrease in sensitivity when the humidity in the environment of the crystals becomes low.
The disadvantage that the decrease in sensitivity by the elimination of water molecules with the humidity decrease may result in a problem of a difference between the densities of the resulting images outputted under a usual humidity condition and under a dry and low humidity condition in the case where the phthalocyanine crystals are used as photographic photoreceptors in laser printers, copying machines, and the like. Particularly, in full-color laser printers and copying machines come into wide use in recent years, the decrease in image density remarkably appears in color tone change of full-color images and the like and hence becomes a serious problem.
As explained above, phthalocyanine crystals containing oxytitanium phthalocyanines having the particular crystal form exhibit high sensitivity but have a problem that the properties remarkably change depending on change in a use environment.
On the other hand, there has been reported V-form hydroxygallium phthalocyanine as a charge generation substance with little change in electrical properties for a humidity change. This V-form hydroxygallium phthalocyanine has an advantage of very little fluctuation in sensitivity for a humidity change but is poor in sensitivity as compared with the oxytitanium phthalocyanine having a particular crystal form and hence it is current situation that electrical properties are insufficient for the requirement for recent high-speed image-forming devices where a number of sheets are printed per unit time in full color (see Non-Patent Document 3).
Furthermore, in order to suppress the sensitivity change of the oxytitanium phthalocyanine having a particular crystal form for a humidity change, a method of adding a moisturizing agent to the charge generation layer has been reported (see Patent Documents 10 to 12). However, in these technologies, only humidity dependence according to residual potential is improved but the sensitivity fluctuation for a humidity change is not sufficiently improved. Since image deterioration depending on a humidity change tends to occur not in solid black images but in halftone images, it is necessary to reduce fluctuation of sensitivity. Actually, in a photo-induced discharge curve of the oxytitanium phthalocyanine having a particular crystal form, it is understood that fluctuation of potential part according to halftone (absolute value of potential is around from 100 to 300 V) is large for a humidity change and thus it is still insufficient for the requirement of reducing the potential fluctuation.
Non-Patent Document 1: Journal of the Society of Electrophotography of Japan, Vol. 29, No. 3, pp. 250-258.
Non-Patent Document 2: Journal of the Society of Electrophotography of Japan, Vol. 32, No. 3, pp. 282-289.
Non-Patent Document 3: Fuji Xerox Technical Report No. 12 1998
Patent Document 1: JP-A-61-217050
Patent Document 2: JP-A-62-67094
Patent Document 3: JP-A-63-366
Patent Document 4: JP-A-2-8265
Patent Document 5: JP-A-63-20365
Patent Document 6: JP-A-3-54265
Patent Document 7: JP-A-3-54264
Patent Document 8: JP-A-3-128973
Patent Document 9: JP-A-3-9962
Patent Document 10: JP-A-2003-207912
Patent Document 11: JP-A-2003-186217
Patent Document 12: JP-A-2003-215825